1. Field of the Invention
The present invention relates to purifying fluids using an electrolytic cell and particularly to purifying fuels for use in fuel cell systems.
2. Description of the Related Art
Catalysts are employed in various commercial and industrial applications to increase chemical reaction rates and are frequently used in the purification of various fluids (e.g., in removing pollutants from the exhaust from automobiles). Electrocatalysts are catalyst materials that increase the rate of the half cell reactions that occur at an electrode in an electrochemical cell. Often, a given material may serve as a catalyst material for many different chemical and/or electrode reactions (e.g., platinum). Herein, the term catalyst will be used specifically in reference to a chemical reaction, as opposed to an electrode or half cell reaction, while the term electrocatalyst will be used in reference to an electrode reaction.
Recently, it has been noticed that the activity of certain catalysts can be enhanced using electrochemical methods known as nonfaradaic electrochemical modification of catalytic activity (NEMCA) or electrochemical promotion (EP). The activity of such catalysts can be increased substantially by incorporating them in the vicinity of an electrode in an appropriate electrochemical cell and then operating the electrochemical cell. Further, the selectivity of such catalysts may be significantly altered (i.e., the relative rates at which competing reactions occur at the catalyst may be significantly changed too). It is hypothesized that catalyst activity/selectivity is promoted by the presence or spillover of certain promoting ionic species generated during the operation of the electrochemical cell.
It has also been noticed that the activity of certain catalysts can be enhanced by electrical activation methods, for instance by passing an appropriate electrical current through the catalyst. Again, the use of such methods can increase the activity of such catalysts substantially. The reasons for such enhancement are not fully understood but may relate in part to the effects of resistive heating of the catalyst (e.g., from heat treatment or localized increases in temperature).
Generally, it is desirable to be able to enhance catalyst activity since such materials typically are in short supply and thus are expensive. In applications where competing reactions can take place, it is also generally desirable to enhance the selectivity of the catalyst for the desired reaction. An exemplary application is the selective oxidation of carbon monoxide.
Carbon monoxide is an undesirable impurity found in the fuel supply or processed fuel contemplated for use in certain fuel cell systems. While high temperature fuel cell types such as the solid oxide or molten carbonate systems can tolerate relatively high levels of CO, low temperature fuel cell types such as the phosphoric acid or solid polymer electrolyte systems are sensitive to CO in the fuel. In solid polymer electrolyte fuel cells in particular, the presence of CO at levels of order of 10 ppm or higher can poison the typical catalyst used in the fuel cell anodes and adversely affect fuel cell performance.
Pure hydrogen gas is a preferred fuel for solid polymer electrolyte fuel cell systems, but is presently difficult to store and handle. Thus, instead of pure hydrogen, a more readily stored and handled hydrocarbon fluid (e.g., methane or methanol) is often used as a fuel supply. The hydrocarbon fuel supply is then chemically processed or reformed to generate hydrogen on demand for the fuel cell system. The processed fuel or reformate typically contains significant quantities of other by-products though along with hydrogen. For instance, methanol reformate obtained via the steam reformation of methanol typically contains about 65% to about 75% hydrogen, about 10% to about 25% carbon dioxide, and from about 0.5% to about 20% by volume of CO, all on a dry basis and, in addition, also contains water vapor. The reformate is thus typically processed further to reduce the CO content. A water/gas shift reactor (a chemical reactor employing catalysts) may be used to react CO impurity with water (producing carbon dioxide and hydrogen) thereby reducing the CO content to about 0.2%-2% by volume, on a dry basis. Then, a selective oxidizer unit (another chemical reactor employing catalysts) may be used to selectively react remaining CO with a small amount of injected oxygen (producing carbon dioxide) and thereby further reduce the CO level. However, the selectivity of such a unit is typically not so high and thus a significant excess of oxygen is needed to oxidize the CO impurity. This excess oxygen can instead react with the fuel itself, representing a loss and inefficiency. Even after such additional treatment, the remaining CO level in the reformate stream may still be undesirably high. Further, the additional processing equipment increases system complexity and adds to its weight, size, and cost.
Other methods have been suggested in the art for reducing the CO levels in reformate. For instance, pressure swing adsorption and membrane filtration methods have been contemplated. Additionally, several methods employing electrochemical processes and electrolytic cells have been suggested in the prior art. In WO 00/16880, a technique is disclosed in which CO is removed from a reformate stream via chemisorption on the anode material of an electrochemical cell. The cell is regenerated from time to time in order to remove chemisorbed CO thereby avoiding saturation of the anode material. The regeneration involves an electrochemical process and can be performed using the cell in either electrolytic or galvanic mode. Alternatively, an electrochemical cell with a proton conducting membrane may be employed as a “hydrogen filter” to produce a CO-free fuel stream from reformate. Operating electrolytically, hydrogen in the reformate may be oxidized at the anode, transported as hydrogen ions through the electrolyte to the cathode, and then reduced back to hydrogen gas at the cathode (i.e., hydrogen is electrochemically pumped across the membrane). The hydrogen obtained from the cathode is thus free of the CO and other impurities in the reformate. However, this process is quite energy intensive and thus may not be a suitably efficient method for practical fuel cell systems.
While many approaches have been investigated for reducing the CO levels in reformed fuel for use in fuel cell systems, there is still a demand for more efficient and less complex methods.